Certain refractory metals, such as tantalum and niobium, can be difficult to isolate in their pure (or primary) form due in part to the thermodynamic stability of precursors thereof, such as oxides. The production of primary refractory metals is desirable because they are used in such applications as raw materials from which capacitor anodes may be prepared. Existing methods of forming primary refractory metals typically involve multi-stage processes in which a refractory metal oxide (e.g., tantalum pentoxide or niobium pentoxide) or other precursor (e.g., tantalum halides) is reduced through one or more steps followed by further refining and purification steps. Such multistage processes typically result in the formation of co-product waste streams.
Raw materials from which tantalum metal may be produced include, for example, heptafluorotantalate (K2TaF7), tantalum halides and tantalum pentoxide. The reduction of potassium heptafluorotantalate with sodium is a known older method of producing tantalum metal. Potassium heptafluorotantalate and small pieces of sodium are sealed in a metal tube, and heated to an ignition temperature which results in the formation of a solid mass that includes tantalum metal, potassium heptafluorotantalate, sodium and other co-products. The solid mixture is then crushed and leached with dilute acid to isolate the tantalum metal, which is typically less than pure.
Tantalum metal may also be formed by a further method in which a molten composition of potassium heptafluorotantalate is reduced in the presence of a diluent salt (e.g., sodium chloride) by the introduction of molten sodium metal into the reactor, under conditions of constant stirring. The molten sodium reduction process results in the formation of a solid mass containing tantalum metal, sodium fluoride, potassium fluoride and other co-products. The solid mass is crushed and leached with a dilute acid solution, to isolate the tantalum metal. Typically, additional process steps, such as agglomeration, must be performed on the product tantalum metal for purposes of improving physical properties. See, for example, U.S. Pat. No. 2,950,185.
The electrolytic production of tantalum involves electrolyzing a molten mixture of potassium heptafluorotantalate containing tantalum pentoxide (Ta2O5) at about 700° C. in a metal container. The electrolytic reduction results in the formation of a solid mass containing tantalum metal, potassium heptafluorotantalate, tantalum oxides and other co-products. The solid mass is then crushed and leached with dilute acid to isolate the tantalum metal, which is typically less than pure. Such electrolytic methods of producing tantalum metal typically are not presently used on a manufacturing scale.
Other methods of producing refractory metals, such as tantalum metal, include the reduction of tantalum pentoxide with calcium metal in the presence of calcium chloride as described in, for example, U.S. Pat. No. 1,728,941; and the reduction of tantalum pentoxide in the presence of a silicide, such as magnesium silicide and a hydride, such as calcium hydride, as described in, for example, U.S. Pat. No. 2,516,863. Such other methods involve multiple stages and result in the formation of co-products from which the refractory metal must be separated.
A more recent method of producing refractory metals, such at tantalum metal, involves less than completely reducing a refractory metal oxide (e.g., tantalum pentoxide or niobium pentoxide) by contacting the refractory metal oxide with a gaseous reducing agent, such as gaseous magnesium. The less than completely reduced refractory metal is then leached, further reduced and agglomerated. See for example, U.S. Pat. No. 6,171,363 B1.
Another recent method of producing refractory metals, such as tantalum and niobium, involves first passing hydrogen gas through powder refractory metal oxide (e.g., tantalum pentoxide) thereby producing an intermediate refractory metal suboxide (e.g., tantalum mono-oxide). In the second stage, the refractory metal suboxide is reduced by contact with a gaseous reducing agent (e.g., gaseous magnesium). The nearly fully reduced refractory metal is then leached, further reduced and agglomerated. See for example, U.S. Pat. No. 6,558,447 B1.
Still further methods of preparing refractory metals involve introducing a refractory metal halide (e.g., tantalum pentachloride) or a refractory metal alkoxide (e.g., tantalum alkoxide) into a plasma formed from hydrogen gas. Such plasma methods result in the formation of undesirable co-products, such as corrosive gaseous hydrogen halides (e.g., gaseous hydrogen chloride), and gaseous alkanols. Refractory metal halide plasma methods are described in further detail in, for example, U.S. Pat. Nos. 3,211,548; 3,748,106; and 6,689,187 B2. Refractory metal alkoxide plasma methods are described in further detail in, for example, U.S. Pat. No. 5,711,783.
U.S. Pat. No. 5,972,065 discloses purifying tantalum by means of plasma arc melting. In the method of the '065 patent, powdered tantalum metal is placed in a vessel, and a flowing plasma stream formed from hydrogen and helium is passed over the powdered tantalum metal.
European Patent Application No. EP 1 066 899 A2 discloses a method of preparing high purity spherical particles of metals such as tantalum and niobium. The method disclosed in the '899 application involves introducing tantalum powder into a plasma formed from hydrogen gas. The temperature of the plasma is disclosed as being between 5000 K and 10,000 K in the '899 application.
It would be desirable to develop methods of preparing substantially pure refractory metals, such as primary refractory metals, that do not involve multiple process steps, and preferably involve only a single reduction step. It would also be desirable that such newly developed methods of refractory metal preparation: make use of feed stocks that are readily available and comparatively safe to handle; and at least minimize the formation of undesirable co-products that must be separated and/or otherwise further processed.